The range state of the <input> element (<input type="range">) has been implemented, behind the preference dom.experimental_forms_range, only enabled by default on Nightly and Aurora channel (バグ 841948).

The support for the <template> element, part of the Web component specification has been implemented (バグ 818976).

The scoped attribute has been added to the <style> element. It allows to include styles that are isolated from the rest of the document. Such styles can be selected using the :scope CSS pseudo-element introduced in Firefox 20. (バグ 508725).

Experimental support for the :scope pseudo-class has been added. Enabled by default in Aurora and Nightly, it can be enabled in release and beta version by setting the layout.css.scope-pseudo.enabled about:config preference to true (バグ 648722).

Support for the viewport-relative <length> units, vh, vw, vmin, and vmax, has landed (バグ 503720).

The -moz-initial value has been unprefixed (バグ 806068). -moz-initial will be kept for a while as an alias; however, authors are strongly encouraged to switch over to initial.

The CSS text-transform property now supports the full-width keyword, which allows a more seamless inclusion of Latin characters in text using ideographic fixed-width characters, like Chinese or Japanese (バグ 774560).

The option to disable JavaScript, including the options to allow moving windows/replace context menu, have been removed. You may still disable JavaScript by double clicking the "javascript.enabled" option in about:config.

Asm.js optimizations are enabled, making it possible to compile C/C++ applications to a subset of Javascript for better performance.

DOM/API

Firefox OS-specific:

Device Storage API: When getting a cursor callback from navigator.getDeviceStorage("sdcard").enumerate, this.done was undefined in Firefox <1.2 (see バグ 902565). This is now fixed. The referenced bug discusses possible compatibility checks/workarounds for this issue.

The RTCPeerConnection Interface from WebRTC is implemented, but currently disabled by default in B2G, as it is not yet functional on Firefox OS devices (see バグ 942343.)

Support for the multipart property on XMLHttpRequest and multipart/x-mixed-replace responses in XMLHttpRequest has been removed. This was a Gecko-only feature that was never standardized. Server-Sent Events, Web Sockets or inspecting responseText from progress events can be used instead.

Until now, when the same headers were repeatedly set with XMLHttpRequest.setRequestHeader, the last-specified value was used. This behavior has been changed to comply with the spec, so those values will be properly combined (バグ 819051).

HTMLMediaElement supports now playbackRate (both read and write), with pitch correction. Pitch correction can be controlled using the property mozPreservesPitch (バグ 495040).

CSSOM: On CSSRule the constant CSSRule.MOZ_KEYFRAME_RULE and CSSRule.MOZ_KEYFRAMES_RULE have been unprefixed to CSSRule.KEYFRAME_RULE and CSSRule.KEYFRAMES_RULE. The prefixed version are temporarily kept for helping Web author to transition their code バグ 816431).

The <semantics> element now determines the visible child as described in the MathML3 specification.

To help MathML authors debugging "invalid-markup" errors in their documents, MathML parsing errors (such as having too many / too few child elements) and warnings about deprecated attributes or wrong attribute values are now reported to the Error Console.

The scriptminsize attribute now accepts unitless values and percent values. They are interpreted as multiples of the default value ("8pt").

Unitless values are now also allowed for the mathsize and fontsize attributes; they multiply the default value.

ネットワーク

Support for the spec-compliant Content-Security-Policy HTTP header (in addition to the experimental X-Content-Security-Policy) has been added (バグ 783049). Note: the patch for this new header landed in Firefox 21, it is disabled on builds (バグ 842657).